The production of elongate profiles from one or more mixtures of rubber or thermoplastics materials utilising an extrusion head to which the mixture or mixtures are supplied by one or more extrusion devices is known. It is also known to cool the profile by conveying it through a cooling zone comprising a plurality of conveyor belts. An apparatus used for producing a profile in the form of a thread strip from two different rubber mixtures is disclosed in German Patent Specification No. 2 201 631. In such prior specification, two separate screw presses, which are disposed one above the other, feed different rubber mixtures to an extrusion head where the material mixtures are combined and extruded therefrom in the form of a tread strip. U.S. Pat. No. 2,382,177 also shows such an arrangement.
The extruded tread strip is then conducted over a tread strip weighing device. Such weighing is achieved by weighing the amount of material located on a rotating conveyor belt. The desired weight of the tread strip is known and if the actual weight deviates therefrom, a certain degree of correction can be achieved. Thus, if the actual weight is higher than the desired weight, the rate of transfer of the tread strip can be increased. This effectively causes a weight reduction to be achieved because the strip is elongated.
In practice, therefore, such apparatuses for producing tread strips have been operated at discharge rate which is slightly in excess of a desired rate. The extruded tread strip is then brought to its desired weight as a result of the elongation.
The only regulation of the characteristics of the tread strip profile offered by such an arrangement is that the extruded tread strip can be elongated or compressed by increasing or reducing the rate of transfer of the strip. However, the rate at which the tread strip is discharged from the nozzle must be considered as the minimum rate for the tread strip transfer apparatus and, for the purpose of regulating the profile thickness, it is vital that the rate of transfer does not fall below this minimum rate.
Simple regulation or control in this manner using a profile strip weighing device which effects a comparison with a desired weight is, in practice, too slow. If, for example, the weighing device ascertains that the tread strip is too heavy, a long piece of unusable tread strip has already been produced. Moreover, a considerable amount of profile material is wasted before the error is corrected.
However, a greater disadvantage resides in the fact that a profile strip weighing device only measures the sum of the extrusion outputs and not the individual outputs of each extrusion device. If, for example, the extrusion device which extrudes the material which is to form the sidewall of a vehicle tyre discharges too much material, the strip weighing device ascertains that too much material is present and the rate of transfer is increased.
It is true that, after some time, this causes the weight of the tread strip per unit length to return to its desired value. However, this is at the cost of reducing the amount of material extruded from the other extrusion device, which material forms the tread surface of the vehicle tyre.
Accordingly, despite the fact that the tread strip has the desired overall weight per unit length, the ratio of the individual mixtures present therein vary considerably and this may cause the vehicle tyre produced using such a tread strip to be considerably weakened.
Since tread strips for vehicle tyres are usually formed from three or more different mixtures, merely ensuring that the tread strip has a desired weight per unit length by utilising a strip weighing device, even if operating satisfactorily cannot possibly ensure that the weight ratios of the individual mixtures present therein are correct.
It has therefore been necessary to attempt to find a way in which both the total weight of the tread strip and the quantities of the individual rubber mixtures from which it is made can be monitored and controlled.
U.S. Pat. No. 4,097,566 also discloses a method and apparatus for producing profiles formed from a plurality of mixtures. The profile strip emerging from the profile strip nozzle, in such prior art arrangement passes a thickness measurement device and is then fed to a conveyor belt after a 90.degree. change in the direction of movement of the strip. The width of the deflection loop of the profile strip is measured. After passing a compensating roller, the profile strip is then guided through a device which measures its weight per unit length and onto a conveyor belt. The conveyor belt is located downstream of the weighing device and the speed of the belt is controllable. By altering the speed of the conveyor belt in dependence upon the weights which have been ascertained, modifications of the strip are caused to occur. Thus, if the measured weight is too high, the profile strip is caused to be elongated by the conveyor belt and if too low, the strip is compressed by the conveyor belt, so that, in either case, the desired weight per unit length of the strip is attained. The method which is described therein is highly complex having regard to the regulating techniques employed and, additionally, has one fundamental disadvantage. If, for example, one of the weighing devices or one of the measuring devices used for measuring the width of a deflection loop ascertains that the produced profile does not have the desired dimensions, adjustment can still only be effected by either extending or compressing the profile strip which is formed from two mixtures.
Each extension or elongation operation on the profile strip, produced, for example, by accelerating the conveyor belts, provides the profile strip with the correct weight but with totally inaccurate and uncontrolled dimensions. These prove to be weak points in later use.
An additional disadvantage of such a known arrangement resides in the fact that the apparatus used is very long. Accordingly, from the time when the deviation from the desired weight of the profile strip is ascertained to the time of its correction, a large amount of unusable material has passed through the apparatus. This is because a large amount of profile strip passes between the particular measuring point and the source of error, that is to say, the point of deviation from the desired value. Such profile strip portion is, therefore, useless.